1. Field of the Invention
This invention relates generally to a method for the catalytic purification of crude terephthalic acid and to the catalyst system employed therein, and more particularly concerns a layered catalyst bed and its use in such purification.
2. Discussion of the Prior Art
Polymer grade or "purified" terephthalic acid is the starting material for polyethylene terephthalate, which is the principal polymer for use in the manufacture of polyester fibers, polyester films, and resins for bottles and like containers.
Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in Meyer, U.S. Pat. No. 3,584,039 or Stech et al., U.S. Pat. No. 4,405,809. In the purification process, the crude terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst containing a noble metal, typically palladium, on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915. This hydrogenation step converts the various color bodies present in the crude terephthalic acid to colorless products.
However, even after purification, the purified terephthalic acid product contains color bodies. It is highly desirable to reduce the concentration of such color bodies that remain in purified terephthalic acid. The color level of purified terephthalic acid product is generally measured directly either by measuring the optical density of solutions of purified terephthalic acid or the b*-value of the solid purified terephthalic acid itself. The optical density of purified terephthalic acid is measured as the absorbance of light at 340 nanometers in its basic solution in a solvent such as sodium hydroxide or ammonium hydroxide.
The measurement of the b*-value of a solid on the Hunter Color Scale is described in Hunter, The Measurement of Appearance, Chapter 8, pp, 102-132, John Wiley & Sons, N.Y., N.Y. (1975), and in Wyszecki et al., Color Science, Concepts and Methods, Quantitative Data and Formulae, 2d Ed., pp. 166-168, John Wiley & Sons, N.Y., N.Y. (1982).
More specifically, the b*-value of purified terephthalic acid can be determined using, for example, a Diano Match Scan Spectrophotometer as follows. Purified terephthalic acid is pressed into a pellet having a thickness of about 0.25 inch and a diameter of about 1 inch. The pellet is then irradiated with white light that has been UV-filtered. The spectrum of the visible light reflected from the sample is determined and tristimulus values (X, Y. and Z) are computed using the CIE Standard Observer functions. Using the weighted-ordinate method, tristimulus values are obtained from the following equations: ##EQU1## where R.sub..lambda. is the percent reflectance of the object at wavelength .lambda. and x.sub..lambda., y.sub..lambda., and z.sub..lambda. are the Standard Observer functions at wavelength .lambda. for CIE Illuminant D65. The tristimulus values, X, Y and Z, identify the color of the object in terms of the mixture of the primary lights that match it visually. Tristimulus values, however, are of limited use as color specifications, because they do not correlate with visually meaningful attributes of color appearance and are not uniform in the spacing of colors as related to visual differences. As a result, "Uniform Color Scales" (UCS) have been adopted which use simple equations to approximate visual response. The UCS scale used by the Diano instrument is the CIE 1976 L*a*b* formula which converts tristimulus values to L*, a*, and b* values as shown below:
L*=25(100Y/Y.sub.o).sup.1/3 -16 PA1 a*=500[(X/X.sub.o).sup.1/3 -(Y/Y.sub.o).sup.1/3 ] PA1 b*=200[(Y/Y.sub.o).sup.1/3 -(Z/Z.sub.o).sup.1/3 ]
The L*-value is a measure of the luminosity or whiteness of an object where L*=100 is pure white, L*=0 is black, and in between in gray. The L*-value is strictly a function of the tristimulus Y-value. The b*-value is a measure of the yellowness-blueness attribute where positive b*-value represent yellow appearance and negative b*-values represent blue appearance. The b*-value is a function of both tristimulus values Y and Z.
Furthermore, even after purification, the purified terephthalic acid product often contains impurities which fluoresce at wavelengths of 390-400 nanometers upon excitation at wavelengths of 260-320 nanometers. Further reduction of such fluorescence of the purified terephthalic acid product is highly desirable. Since the concentration of such impurities in purified terephthalic acid can vary significantly, specifications are often established for the amount of such fluorescence which can be permitted for the purified terephthalic acid product. The problem of the control of such fluorescence by purified terephthalic acid is complicated because some of the fluorescent impurities are soluble and can be removed by conventional procedures for purifying terephthalic acid while other such fluorescent impurities are insoluble and cannot be removed by such conventional procedures. Furtheremore, upon chemical reduction during purification of crude terephthalic acid, some impurities which do not themselves fluoresce at wavelengths of 390-400 nanometers upon excitation at wavelengths of 260-320 nanometers are converted to their reduced forms which fluoresce at 390-400 nanometers upon excitation by wavelengths of 260-320 nanometers.
Puskas et al., U.S. Pat. Nos. 4,394,299 and 4,467,110 disclose the use of a combination noble metal catalyst, for example, a palladium/rhodium catalyst on a porous carbonaceous surface, for purification of aqueous terephthalic acid solutions. These two patents also show the use of a rhodium-on-carbon catalyst under reducing conditions and review various heretofore known methods of preparing a Group VIII metal catalyst having activity and selectivity suitable for the purification of terephthalic acid by hydrogenating its principal impurity, 4-carboxybenzaldehyde, to p-toluic acid.
However, p-toluic acid is also an impurity that must be removed from the hydrogenated aqueous terephthalic solution. While such removal can be achieved to a large extent owing to the greater solubility of p-toluic acid as compared to terephthalic acid, in water nevertheless substantial amounts of p-toluic acid are trapped within purified terephthalic acid crystals as the hydrogenated terephthalic acid solution is crystallized to recover purified terephthalic acid.
To avoid the disadvantages attendant to the separation of p-toluic acid, it has been proposed to decarbonylate 4-carboxybenzaldehyde in aqueous solutions to benzoic acid in the presence of a palladium-on-carbon catalyst but without the simultaneous hydrogenation of the other impurities that may be present in aqueous solutions of crude terephthalic acid since benzoic acid is more soluble in water then p-toluic acid. See, for example, Olsen, U.S. Pat. No. 3,456,001.
However, the foregoing decarbonylation of 4-carboxybenzaldehyde to benzoic acid produces equimolar amounts of carbon monoxide, a well-known poison for the noble metals such as palladium (see, for example, Kimura et al., U.S. Pat. No. 4,201,872). In any attempt to minimize catalyst poisoning, Kimura et al., in the aforementioned patent, propose to carry out the decarbonylation at relatively low process pressures so as to minimize dissolved carbon monoxide concentration in the liquid reaction medium. The process pressures also must be controlled within a closely defined pressure range. The generated carbon monoxide is purged from the reactor as a gas.
We have now discovered that the use in the aforesaid purification of crude terephthalic acid of a catalyst system comprising a first layer of catalyst particles containing a metal of Group VIII of the Periodic Table of Elements having a single electron in its outermost orbital when in the ground state supported on a carbon carrier and a second layer of palladium-on-carbon catalyst particles and the passage of the aqueous solution of crude terephthalic acid through the aforesaid first layer of rhodium-on-carbon catalyst particles and then through the second layer of palladium-on-carbon catalyst particles permits the amount of p-toluic acid produced during purification of crude terephthalic acid to be minimized. Such method of using the aforesaid catalyst system does not promote the hydrogenation of 4-carboxybenzaldehyde to p-toluic acid but instead promotes the decarbonylation of 4-carboxybenzaldehyde to benzoic acid, which is more soluble than p-toluic acid in water and thus is more readily separable than p-toluic acid from terephthalic acid upon crystallization of the terephthalic acid. This permits a feed solution having a relatively higher 4-carboxybenzaldehyde content to be processed economically. In addition, such benefits are attained without the aforesaid deleterious effects on the catalyst system caused by the generated carbon monoxide.
Furthermore, we have also discovered that such method of using the aforesaid catalyst system effects a further decrease in the concentration of color bodies and of fluorescent impurities in the resulting purified terephthalic acid, relative to the use of a conventional palladium-on-carbon catalyst alone.